Hello, I hope the following assists you.
Other than the pipes you've replaced the only other common point of air ingress into the fuel system is the injector return pipes, most of the time though if there is a concern with these you tend to see a little diesel around the injectors..
There are a couple of things you can try to help pinpoint where the air is getting in.
Try running the engine on a fuel supply from a can of diesel and pipe the return into the can as well, put the supply direct onto the fuel filter this way you can eliminate any concern with the tank pick up and connectors. Run the car on the tank then leave it to stand as you normally would, if the car starts after been left then you know there is a problem with the pick up in the tank or its connectors.
You can also try a clear piece of pipe at various point in the low pressure fuel system and see if there is any air in the fuel. The main point to remember is that air going into the return will cause a problem as it is cycled back though the pump so does allow the air to get into the feed side of the system.
In all honesty it's not very often you do see a diesel leak when air is getting into the fuel system, air tends to be drawn in by the vacuum created by the fuel flow.
If you have access to a vacuum gauge you can see if various parts of the system hold a vacuum.
I hope this helps but if you have more questions regarding this let me know and I'll reply as soon as I'm able to.